The general problem of dark current field shading in CCD imagers of field transfer type and provisions for compensating against this undesirable effect are described in U.S. Pat. No. 4,496,982 issued Jan. 29, 1985 by P. A. Levine, entitled "Compensation Against Field Shading in Video from Field-transfer CCD Imagers", assigned to RCA Corporation, and is incorporated herein by reference. Field shading is a variation in black level across the field in the direction of line advance. It is caused by successive lines of charge packets representative of image samples spending progressively longer periods of time in the field storage register of a field-transfer type of CCD imager. The increased length of time each line of charge packets spends in the field storage (or B) register allows a longer time for the accumulation of dark current and thereby shifts the black level for each successive line further towards white. Field shading is superposed on an optical black, dark current pedestal attributable to the accumulation of dark current in the image (or A) register and in the output line (or C) register.
The rate of dark current accumulation increases with increased temperature of the semiconductive substrate on which the imager is formed. It is desirable to develop a reference voltage that is proportional to dark current accumulated during field trace in the CCD imager, from which signals to compensate against field shading can be developed that provide good compensation despite changes in substrate temperature. Such a reference voltage is more likely to provide good compensation without need for readjustment if it is derived as a direct measure of an accumulation of dark current in an area of the same substrate as that the imager itself is constructed upon. The temperature-sensitive reference voltage can also be used to control cooling of the CCD imager to maintain dark current levels constant as described by P. A. Levine in U.S. patent application Ser. No. 571,719, filed Jan. 18, 1984, entitled "Dark Current Level Regulation in Solid-state Devices" and assigned to RCA Corporation.
The integral of remnant charge left in the field storage register after field trace can, as described in U.S. Pat. No. 4,496,982, be removed during field retrace and Riemann integrated over time to develop a reference voltage that is a measure of dark current accumulation during imager field-trace. The area of the substrate over which remnant charge in accumulated is that of the field storage register, so the sensitivity of the measure of dark current accumulation is good. The practical problem that one runs into in practicing this method is that the opaque mask over the field storage and output line registers used to shield them from illumination does not, at least in imagers presently commercially available, adequately preclude light from the rows of the masked field storage register close to the unmasked image register. The admission of this light causes photoresponse in these rows of the field storage register. This photoresponse is clocked forward during the line-by-line advance of the charge packets through the field storage register in the field trace interval, thence to appear in every line of remnant charge clocked out of the field storage register during field retrace interval.
In color television cameras, where gamma-correction is used, small video variations close to black level are stretched about five times respective to video variations in brighter portions of the televised scene. The photoresponse adulterates to an unacceptable extent the measurement of accumulated dark current made by integrating remnant dark currents removed from the B register during field retrace. Even apparatus for suppressing photoresponse in remnant dark current from the field storage register, by taking a slope measurement thereof, does not provide an accumulated dark current measurement sufficiently free of photoresponse. Such apparatus has been described by R. F. Wood, Jr., J. F. Monahan and P. A. Levine in U.S. Pat. No. 4,525,743 issued June 25, 1985; entitled "Dark Current Measurement and Control for Cameras Having Field-transfer CCD Imagers" and assigned to RCA Corporation.